Development of adaptive tests used in K-12 settings requires the creation of stable measurement scales to measure the growth of individual students from one grade to the next, and to measure change in groups from one year to the next. Accountability systems like No Child Left Behind require stable measurement scales so that accountability has meaning across time. This study examined the stability of the measurement scales used with the Measures of Academic Progress. Difficulty estimates for test questions from the reading and mathematics scales were examined over a period ranging from 7 to 22 years. Results showed high correlations between item difficulty estimates from the time at which they where originally calibrated and the current calibration. The average drift in item difficulty estimates was less than 0.01 standard deviations. The average impact of change in item difficulty estimates was less than the smallest reported difference on the score scale for two actual tests. The findings of the study indicate that an IRT scale can be stable enough to allow consistent measurement of student achievement. (Contains 8 figures.)